Devil’s thorn threatens some of our most scenic locales — here’s a chance to root it out

As nefarious weeds go, devil’s thorn is a “10 Most Wanted” sort of goon.

The federal government has branded it a noxious invasive, and it’s earned the equivalent designation from several nations around the world.

Over the past few years, devil’s thorn has been quietly infiltrating the Central Coast.

In Morro Bay, it’s been found at the campground, around the natural history museum, along the Cloisters trails and boardwalk, near the top of Black Hill, and at the edges of Morro Strand State Beach. In Los Osos, it has invaded Montaña de Oro, appearing on dune trails and the sand spit and in Horse Camp; because those spiny seeds easily latch onto horses’ feet, this puts Montaña de Oro’s pristine backcountry in jeopardy. The weed also crops up periodically along Los Osos Valley Road. Most troublingly, it has taken hold in the Morro Dunes Ecological Reserve, the new network of trails above Highland Drive that connects to Montaña de Oro. In the adjacent neighborhood of Redfield Woods, some 700 plants were found on 10 private lots.

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In appearance, devil’s thorn bears a loose resemblance to New Zealand spinach, a fairly benign weed in our area. Its leaves are roughly spade-shaped, usually with wavy edges, and the stems are often tinged with red. Those hard, prickly seeds cluster in the leaf forks along every stem, with another group studding the base of the plant. (More later on those basal seeds and their role in an eerie disappearing act.)

Known to biologists as Emex spinosa, devil’s thorn also goes by other names: prickly dock, doublegee and three-corner jack. That last one sounds like such fun — a card game, a magic trick? — but this weed is no joke, and its spread turns out to matter more than you might think.

As with other invasive plants that threaten the Central Coast’s ecology, devil’s thorn is drought-tolerant and thrives in sand. An annual, it grows rapidly, and each plant contains both male and female flowers, so that, quite handily, it can fertilize itself. It produces seeds abundantly, and these seeds can both travel far and lie dormant for several years, while waiting for opportune conditions.

That wanderlust is enhanced by a marvel of engineering: the spiny seeds are constructed so that when they lie on the ground, one thorn is always pointing up. It can thus easily hook onto the feet of passing animals or wedge into the treads of hiking boots. It can also, quite unpleasantly, pierce through thin-soled shoes and puncture bicycle tires.

Residents are urged to dig up devil’s thorn plants and dispose of them. Use a shovel, and keep in mind the phrase “armed and dangerous”: those needle-sharp seeds spike the length of each stem. Wear thick gloves and long sleeves; on a hot day last spring, in a tank top, I pursued a large clump of Emex on the LOVR right-of-way and emerged looking like I’d wrestled with a feral cat.

Extricating the plant’s thick taproot is essential to eradicating this pest, and the reason why is the creepiest thing I’ve ever learned about a plant. At the end of the growing season, the taproot dries out and shrinks back, drawing the seeds at the base of the plant underground — in effect, sowing itself for next year.

Those of us who battle noxious weeds like devil’s thorn often hear the same question: Why bother? Why prefer a native plant to a nonnative one — what difference does it make?

The answer lies in biodiversity. The planet’s vast and dazzling range of animals and plants and microorganisms is what sustains life on earth. But there are threats to this intricate and carefully balanced web. The chief one, as anyone who thinks about it for a minute will realize, is habitat loss. The surprise may lie in the menace that fills the No. 2 slot: invasive species.

And one of the worst is devil’s thorn.

Bonnie Thompson lives in Los Osos, where she has been helping with habitat restoration at Sweet Springs Nature Preserve for the past 10 years.

Join the effort to eradicate devil’s thorn

Celebrate Los Osos is sponsoring a “Pull Together” event Saturday, Feb.17, from 10 a.m. to noon. Anyone interested should meet at the Red Barn, where Celebrate Los Osos will have a list of locations where these weeds are growing. Bring gloves, wear SPF and dress in layers. Large trash bags will be provided, and there will be a dumpster for disposal of weeds.